Why it’s time to let the OSI model die

One of the most enduring models in networking is the famous seven-layer OSI model. X.25 roughly, but pretty accurately, is based on the first three layers. And it stopped there. Subsequently – partly because of technology and partly because of marketing – Layers 4 through 7 have taken on a life of their own in the network as well as the applications.

The lines began to blur more than 20 years ago. Frame relay didn’t exactly fit the model because it contained some, but not all, of both layers 2 and 3. And TCP/IP didn’t even come close to following the seven layers as specified.

As stated in a recent column “Carrier Ethernet and the New OSI” by Tom Nolle of CIMI Corporation: “Despite the longevity of the references to the OSI model, the conception of the OSI model has changed over the years. Some ‘layers’ have been added, and some don’t seem to be getting used very much. Most recently, there is talk about concepts like ‘virtual networks’ and ‘abstract topologies’ that don’t clearly relate to the old OSI concepts. To make matters worse, the Internet’s evolution, based on TCP/IP, never strictly followed the old OSI model at all. A reasonable person might ask whether people who talk about ‘Layer 1’ or ‘Layer 3’ aren’t blowing kisses at an old friend instead of recognizing the relevance of the original OSI model.”

A few years ago, Larry Hettick and Steve attempted to redefine the “real” meaning in today’s world of the seven layers in a paper titled “Data Networking Basics”. However, Tom, in his column proposes a simplified three-level model to replace the traditional seven-layer model.

In this model, Tom calls the first layer the Connection Layer, and according to Tom, “This layer is responsible for information delivery to the user access point.” The Facilitation Layer is used to provide “services to users/applications to enable their use of the Connection Layer and to provide other facilitation for even higher-layer services.” And on top there’s the Application Layer, which is used for “’user’ and ‘application’ services.”

This makes a lot of senses to us. We’ve put the DB-25 connectors into the museum (or the trash), and it’s time to move on to a new architecture as well and to stop trying to shoe-horn terminology for a 30 year-old model to today’s technology.